1. Field of the Invention
The present invention relates to a method and device for shaping the waveform of an optical signal.
2. Description of the Related Art
The modulation rate of a signal used in optical fiber communication continues to increase year after year. At present, a high-speed system having a modulation rate of 2.5 Gb/s or 10 Gb/s per channel is in actual use. Further, optical transmission employing a modulation rate of 40 Gb/s per channel is being examined in a system under investigation.
Various formats such as NRZ and RZ formats are used for an optical signal to increase a modulation rate. Further, an optical amplifier for compensating for loss of an optical signal to increase a transmission distance is in actual use. For example, a rare earth doped optical amplifier characterized in direct amplification, typically such as an erbium doped fiber amplifier (EDFA), is in actual use. A repeater employing such an optical amplifier is used to compensate for a reduction in power of an optical signal due to transmission loss by an optical fiber.
The optical amplifier is an analog amplifier, and it can therefore amplify an optical signal in an arbitrary format. However, in amplifying an optical signal, the optical amplifier always adds amplified spontaneous emission (ASE) to the optical signal, so that the signal-to-noise ratio (SN ratio) of the optical signal is reduced by the amplification.
Accordingly, the SN ratio decreases with an increase in the number of repeaters, causing a limit to a transmission distance. The nonlinear optical-effects and group velocity dispersion in an optical fiber increase the ASE to degrade the waveform of an optical signal, causing a transmission limit.
To break down such a transmission limit, regenerative repeating is required before bit error becomes excessive.
It is estimated that wavelength division multiplexing (WDM) will be extensively applied to a future photonic network. Accordingly, it is expected that different modulation rates of 2.5 Gb/s, 10 Gb/s, and 40 Gb/s, for example, will be mixed for optical signals in WDM channels. Further, the formats of optical signals in WDM channels are considered to be different from each other. Accordingly, such a network is required to employ a regenerative repeater capable of performing a transparent operation independent of the modulation rates and formats of optical signals.
In a regenerative repeater using an electrical circuit, there is a limit such as an electrical band limit, and it is therefore difficult to realize such a transparent operation. In an all-optical regenerative repeater capable of performing all kinds of signal processing in optical level, the above-mentioned limit is almost eliminated to allow a transparent operation.
The functions required for the all-optical regenerative repeater are amplitude restoration or reamplification, waveform shaping or reshaping, and timing restoration or retiming. These functions are referred to as 3R functions, and in particular, the first and second functions are referred to as 2R functions.
As shown in FIG. 1, a waveform shaping device used most generally in the prior art requires electrical signal processing. First, input signal light is converted into an electrical signal by an O/E converter 2. This electrical signal is next input into an electrical 2R regenerator 4 to shape the waveform of the electrical signal. Thereafter, the waveform-shaped electrical signal is converted into an optical signal by an E/O converter 8 using a laser diode (LD) 6.
In this waveform shaping method employing electrical signal processing, the modulation rate of the signal light that can be processed is limited by electrical band limitation. As a result, it is difficult to regenerate the waveform of a high-speed optical signal.
FIG. 2 shows a conventional waveform shaping device designed to optically perform all kinds of signal processing. Signal light controlled in its polarization state by a polarization controller (PC) 10 and probe light from a laser diode (LD) 12 as a probe light source are supplied to an optical gate 14. In the optical gate 14, waveform shaped light is obtained by an optical gate operation. Thereafter, the waveform shaped light output from the optical gate 14 is extracted by an optical bandpass filter (BPF) 16, and output from this device. In this case, the wavelength of the waveform shaped light is equal to the wavelength xcexp of the probe light. Further, the wavelength xcexp of the probe light is set different from the wavelength xcexs of the signal light from a necessity viewpoint of extraction of the waveform shaped light.
Examples of the optical gate 14 include a nonlinear optical loop mirror, a nonlinear switch having a Michelson or Mach-Zehnder interferometer configuration, and a switch employing a saturable absorber.
The prior art shown in FIG. 2 has a problem that polarization dependence occurs in obtaining a sufficient waveform shaping function. As shown in FIG. 2, the polarization controller 10 is required to adjust or control the polarization state of the signal light to be input into the optical gate 14, causing complexity of the device configuration.
It is therefore an object of the present invention to provide a method and device for shaping the waveform of an optical signal independently of the modulation rate and format (pattern) of the optical signal.
It is another object of the present invention to provide a method and device for shaping the waveform of an optical signal independently of the polarization state of the optical signal.
It is a further object of the present invention to provide a method and device for shaping the waveform of an optical signal which can easily simplify a device configuration.
It is a still further object of the present invention to provide a method and device for shaping the waveform of an optical signal without wavelength conversion.
In accordance with an aspect of the present invention, there is provided a method of shaping the waveform of an optical signal, comprising the steps of inputting said optical signal into a first optical gate to suppress a space-level noise of said optical signal; and inputting an optical signal output from said first optical gate into a second optical gate to suppress a mark-level noise of said optical signal output from said first optical gate.
According to this method, the first and second optical gates are cascaded. Accordingly, by using a suitable device as each of the optical gates, at least one of the objects of the present invention can be achieved.
In accordance with another aspect of the present invention, there is provided a device for shaping the waveform of an optical signal. This device comprises first and second optical gates cascaded. The first optical gate receives the optical signal to suppress a space-level noise of the optical signal, and the second optical gate receives an optical signal output from the first optical gate to suppress a mark-level noise of the optical signal output from the first optical gate.
In accordance with a further aspect of the present invention, there is provided a system including an optical fiber transmission line for transmitting an optical signal and at least one waveform shaping device arranged along said optical fiber transmission line. The waveform shaping device is the device according to the present invention.
In accordance with a still further aspect of the: present invention, there is provided a system including an optical demultiplexer for receiving WDM signal light obtained by wavelength division multiplexing a plurality of optical signals to separate said WDM signal light into said plurality of optical signals, a plurality of waveform shaping devices for receiving said plurality of optical signals output from said optical demultiplexer, respectively, and an optical multiplexer for wavelength division multiplexing a plurality of optical signals output from said plurality of waveform shaping devices. Each waveform shaping device is the device according to the present invention.